Method and Apparatus for Purifying Nitration Products

ABSTRACT

The present invention relates to a method for removing impurities from nitrated crude products obtained during the nitration of nitratable aromatic compounds, after removal of the final nitrating acid, by treatment with a washing medium, and also to a plant or apparatus suitable for implementing this method. Further provided by the invention is a production plant for the nitration of nitratable aromatic compounds with subsequent purification of the nitrated products.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a divisional application that claims priority to and the benefitof co-pending U.S. patent application Ser. No. 14/118,737, filed on Nov.19, 2013, which is a National Stage filing of International ApplicationPCT/EP 2011/002139 filed May 18, 2012, entitled “METHOD AND APPARATUSFOR PURIFYING NITRATION PRODUCTS” claiming priority to GermanApplication No. DE 10 2011 102 059.8 filed May 19, 2011. The subjectapplication claims priority to co-pending application Ser. No.14/118,737, to PCT/EP 2011/002139, and to German Application No. DE 102011 102 059.8, the disclosures of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates to the technical field of nitration, inparticular the preparation of nitrated aromatic organic compounds(hereinafter referred to synonymously as “nitroaromatics”, “nitrationproducts” or the like) and their purification after production.

The present invention relates in particular to a process for removingimpurities (in particular unreacted starting materials, reactionby-products, nitrating acid and reaction products thereof, e.g. nitrogenoxides or nitrous acid, etc.) from crude nitrated products obtained inthe nitration of nitratable aromatic compounds by treating the crudeproducts, after removal of the residual nitrating acid, with a washingmedium. In other words, the present invention relates to a process forpurifying crude nitrated products obtained in the nitration ofnitratable aromatic compounds after removal of the residual nitratingacid.

Furthermore, the present invention relates to an apparatus or plant forremoving impurities from crude nitrated products obtained in thenitration of nitratable aromatic compounds after removal of the residualnitrating acid. The apparatus or plant of the invention is, inparticular, suitable for carrying out the process of the invention.

Finally, the present invention relates to a production plant fornitrating nitratable aromatic compounds with subsequent purification ofthe nitrated products.

Aromatic nitro compounds such as nitrobenzene (MNB), mononitrotoluene(MNT), dinitrotoluene (DNT), trinitrotoluene (TNT), nitrochlorobenzene(MNCB), etc., which are prepared by reaction of a corresponding aromaticsuch as benzene, toluene, xylene, chloro-benzene, dichlorobenzenes,etc., with nitric acid, either directly or in the presence of sulfuricacid catalyst and water-binding agent, have to be subjected beforefurther processing to multistage washing and additional purificationsteps in order to remove the impurities which are still dissolved orsuspended in the crude nitroaromatics, e.g. sulfuric acid, nitric acid,nitrogen dioxide, nitrophenols, nitrocresols, etc., which can bepresent, for example, as mononitro, dinitro and trinitro compounds, andother oxidation products such as nitrobenzoic acids and degradationproducts from the decomposition of nitrophenols, or the unreactedaromatics or undesirable isomers, e.g. in the production of TNT, fromthe crude mixture of nitroaromatics.

The washing of the crude nitroaromatics in order to remove the dissolvedand suspended acids of the nitration mixture, the nitrophenols and otheracidic impurities which can still be extracted by the washing mediumusually consists of three steps (see, for example, F. Meissner et al.,Industrial and Engineering Chemistry, Vol. 46, pages 718 to 724 (1954);Ullmanns Enzyklopädie der Technischen Chemie, 4^(th) edition, Vol. 17,pages 384 to 386; H. Hermann et al., “Industrial Nitration of Toluene toDinitrotoluene”, ACS Symposium Series 623 (1996), pages 234 to 249,editors: L. F. Albright, R. V. C. Carr, R. J. Schmitt; U.S. Pat. No.6,288,289 B1; EP 1 816 117 B1). Water is usually used as washing mediumwith washing usually being carried out as a liquid/liquid wash (i.e. attemperatures at which the nitroaromatic to be washed is present asliquid).

This three-stage wash usually comprises the following steps:

-   1. An acid wash with water to remove the dissolved and suspended    mineral acids, e.g. sulfuric acid, nitric acid and nitrogen dioxide    (“acid wash”).-   2. A basic or alkaline wash in the presence of a base (“alkali    wash”), e.g. sodium carbonate (soda), sodium bicarbonate, sodium    sulfite, sodium hydrogensulfite, ammonia, sodium hydroxide,    potassium hydroxide, etc. (see, for example, U.S. Pat. No. 4,482,769    A, U.S. Pat. No. 4,597,875 A or U.S. Pat. No. 6,288,289 B1), to    remove the weakly acidic impurities dissolved in the crude    nitroaromatic, e.g. the nitrophenols, nitrocresols, nitrobenzoic    acids, degradation products from the oxidative decomposition of the    phenols or of aliphatic or cyclic hydrocarbons, etc., e.g. oxalic    acid, etc., or the unsymmetrical isomers in the case of TNT (“basic    wash”).-   3. A neutral wash to remove the residual traces of alkali and to    further reduce the amount of impurities still remaining in traces in    the product (“neutral wash”).

The aim of these washing steps is to obtain not only a pure product butalso very little wastewater per metric ton of product, where thewashed-out impurities are present in the wastewater in such a form thatthey can be disposed of inexpensively.

To minimize the amounts of water required for this wash, the wash can,for example, be carried out in countercurrent in such a way that thewater used for the neutral wash is, after addition of bases, used in thealkali wash (cf., for example, A. B. Quakenbush et al., The OlinDinitrotoluene (DNT) Process, Polyurethanes World Congress 1993,Publish.: Technomic Lancaster, pages 484 to 488) or that the acid washis carried out using a minimal amount of water, so that a concentratedacid which can be recirculated either directly or after furtherconcentration to the nitration is obtained.

Thus, EP 0 279 312 B1, EP 0 736 514 B1 and EP 1 780 195 B1 describeprocesses by means of which the mineral acids still suspended anddissolved in the nitroaromatics after the nitration, e.g. sulfuric acid,nitric acid and nitrogen dioxide, are washed out in a plurality ofstages and selectively and are recirculated to the nitration, so that nowastewater is obtained from the acid wash and has to be disposed of.

However, processes in which, in order to minimize the amount ofwastewater to be treated, no acid wash is carried out but instead onlyan alkaline wash and a neutral wash, as described, for example, inKirk-Othmer, Encyclopedia of Chemical Technology, 4^(th) ed., Vol. 17,pages 136 to 138, or in U.S. Pat. No. 4,091,042 A, have also becomeknown.

Apart from minimizing the waste streams, a further aim is to minimizethe technical outlay required for the wash (e.g. by the technology usedfor washing being specifically matched not only to the washing stage butalso to the product to be washed).

As washing apparatuses, mixer-settler units (cf., for example, EP 1 593654 A1) in which the mixing part is usually a stirred vessel (cf., forexample, Ullmann's Encyclopedia of Industrial Chemistry, 5^(th) ed.,Vol. B 3, pages 6.19 to 6.21; M. Baerns et al., Technische Chemie,Verlag Wiley-VCH 2006, pages 352/352) are usually used in the individualwashing stages for washing the nitroaromatics to be purified. Thus, theGerman patent DE 1 135 425 describes an arrangement of mixers andsettlers which allows even nitroaromatics which are crystalline at roomtemperature, e.g. DNT, TNT or NCB, to be washed in liquid form atelevated temperatures with minimization of the outlay for heating.However, centrifugal pumps and static mixers have also been used asmixers (cf., for example, the documents U.S. Pat. No. 3,221,064 A or EP1 816 117 B1).

However, the use of the mixer/settler technology (cf., for example,FIG. 1) is complicated and expensive. Due to the unavoidable carryoverin the case of continuously operated stirred vessels as mixers, it is,especially in the removal of nitrophenols or nitrocresols when these arepresent in high concen-trations in the crude nitroaromatic, necessary towork in a number of stages and preferably in countercurrent in order toobtain the low content of impurities which is desired for the furtherprocessing of the nitroaromatic (e.g. a content of nitrophenols of lessthan 10 ppm, preferably from 2 to 3 ppm). A wash in multistageextraction columns is also technically complicated and expensive and notvery effective. In addition, the generation of large exchange areas fora two-phase mixture in a short time for effective mass transfer followedby a rapid chemical reaction can be achieved neither in a stirred vesselnor in extraction columns.

J. M. Coulson, F. E. Warner, “A Problem in Chemical Engineering Design:The Manufacture of Mononitrotoluene”, a publication by “The Institutionof Chemical Engineers”, 56, Victoria Street, London S.W.1, 1949, pages25/26, describes a triple wash of the MNT using a washer of theHolley-Mott (mixer/settler) type, in which the acid wash and thealkaline wash is carried out in countercurrent in at least two stages inorder to achieve sufficient removal of the acids and nitrocresolsdissolved or suspended in the MNT.

In the Canadian patent CA 1 034 603, a four-stage acid wash incountercurrent is proposed in order to wash out the nitric acid andsulfuric acid dissolved and suspended in the crude DNT.

U.S. Pat. No. 4,091,042 A describes a four-stage wash using sodiumcarbonate in countercurrent for removing all acidic components fromcrude nitrobenzene, e.g. entrained sulfuric acid and the dinitrophenolsand picric acid dissolved in the nitroaromatic down to 2000 ppm andobtain the desired purity.

EP 1 816 117 A1 describes a four-stage neutral wash in countercurrentusing four stirred vessels and the associated separation apparatuses(known as “mixer/settler technology”) in order to reduce the still toohigh content of nitrophenols after the alkaline wash from about 50 ppmto a residual content of about 2 ppm. However, even when the stirredvessels are replaced by centrifugal pumps as mixing devices, threestages are still required to obtain a residual content of nitrophenolsin the resulting nitrobenzene of 3 ppm.

U.S. Pat. No. 4,994,242 A discloses that static mixers are not suitableas mixing device in two-phase systems on the industrial scale alone toproduce optimal dispersion of the two mutually immiscible phases in oneanother. Thus, EP 1 816 117 B1 describes the use of a static mixer forthe alkaline wash; the nitrobenzene treated therewith still containsmore than 50 ppm of nitrophenols which have to be brought down to about2 ppm by means of a complicated multistage neutral wash.

As has been explained for an acid wash in EP 1 780 195 B1, the washingof nitroaromatics is a complex operation. Apart from generation of asufficiently large exchange area between organic phase and washing phase(usually water) in order to achieve optimal transition of the impurityto be removed from the organic phase, the effectiveness of a washingstage depends on the partition equilibria of the impurity betweenorganic phase and washing medium and also on whether the impurityextracted from the organic phase is stable as such in the washing mediumor is withdrawn from the partition equilibrium by a subsequent reaction.

Thus, nitrogen dioxide reacts with water after transition from theorganic phase into the aqueous phase so as to disproportionate intonitric acid and NO according to equation (1):

3NO₂(=3/2N₂O₄)+H₂O→2HNO₃+NO  (1)

Both the transition of the nitrogen dioxide from the organic phase,probably as dimer, and also the reaction of the nitrogen dioxide (asN₂O₄) with water are comparatively slow reactions compared to aneutralization, so that time is required for removal of the nitrogendioxide from the organic phase by means of a wash with subsequentchemical reaction.

On the other hand, in the case of acids such as sulfuric acid, nitricacid or the weakly acidic nitrophenols, the dissociation of the acidsinto hydronium ions and the associated anions which occurs in thewashing water (equation 2) or the neutralization which occurs in thepresence of alkali (equation 3) is a very rapid process by means ofwhich the washed-out impurities are withdrawn from the partitionequilibrium between nitroaromatic and washing water and are then foundin anionic form only in the washing water.

H₂SO₄+H₂O→H₃O⁺+HSO₄ ⁻  (2)

NO₂Ar—OH+NaOH→NO₂Ar—O⁻Na⁺+H₂O  (3)

As a result of this rapid neutralization of anion-forming materials inthe alkaline washing medium, it is to be expected that the extraction ofthese materials from the organic phase is essentiallymass-transfer-controlled and the wash follows essentially the samekinetic laws as a mononitration, e.g. the nitration of benzene to formnitrobenzene.

The processes and plants known from the prior art for purifying crudenitrated products often do not operate with high efficiency or else notin a satisfactory way. Hitherto, excessively complex process sequencesor operations have been associated therewith, and the desired puritiesare often not achieved, at least not with a justifiable outlay.

BRIEF SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a processand an apparatus or plant for removing impurities from crude nitratedproducts obtained in the nitration of nitratable aromatic compoundsafter removal of the residual nitrating acid, in which theabovementioned problems and disadvantages associated with the prior artare at least largely avoided or else at least reduced in severity.

In particular, it is an object of the present invention to provide aprocess and an apparatus or plant suitable for carrying out this processby means of which efficient purification of the crude nitrated productsas are obtained from the nitration of nitratable aromatic compoundsafter removal of the residual nitrating acids is made possible.

It is a further object of the present invention to carry out the washingof the crude nitroaromatics which result after removal of the residualnitrating acid, in which significant amounts of impurities such asentrained nitrating acid, dissolved sulfuric acid, nitric acid, nitrogendioxide, nitrophenols, nitrobenzoic acids, degradation products from theoxidative degradation of nitrophenols, etc., can be present, effectivelyin a single-stage manner in each washing step in such a way that thenitrophenol content of the washed nitroaromatic is very low (e.g. in thecase of nitrobenzene from an adiabatic nitration originally containingabout 2000 ppm of dinitrophenols and trinitrophenols, the content ofnitrophenols after the alkaline wash is below 50 ppm, preferably below10 ppm, and after the neutral wash is below 2 ppm) and the outlay andcosts are significantly lower than in the case of the previouslyutilized processes and apparatuses of the prior art.

The abovementioned objects are achieved according to the invention by aprocess as described herein; further, advantageous further developmentsand embodiments of the process of the invention are similarly described.

The present invention further provides an apparatus or plant; further,advantageous further developments and embodiments of this aspect of theinvention are similarly disclosed.

In addition, the present invention provides a production plant; further,advantageous embodiments and further developments of this aspect aresubject matter are provided.

It goes without saying that embodiments, variants, advantages or thelike which are referred to below in respect of only one aspect of theinvention, to avoid unnecessary repetitions, of course also applyanalogously to the other aspects of the invention.

Furthermore, it goes without saying that when values, numbers and rangesare indicated below, the respective values, numbers and ranges indicateddo not constitute a restriction; it will be self-evident to a personskilled in the art that the indicated ranges or values can be deviatedfrom in individual cases or for a particular application without goingoutside the scope of the present invention.

In addition, all values and parameters or the like indicated below canbasically be measured or determined by standardized or explicitlyindicated methods of determination or else by methods of determinationwith which a person skilled in this field will be familiar.

This having been made clear at the outset, the present invention will bedescribed in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a schematic depiction of a wash of nitroaromaticsaccording to the prior art by means of mixer/settler technology for theusual three washing stages of a wash of nitroaromatics;

FIG. 2 provides a schematic depiction of a single-stage wash fornitroaromatics according to the process of the invention or using theapparatus or plant of the invention;

FIG. 3 provides a schematic flow diagram of the process of the inventionor a schematic depiction of the apparatus or plant of the invention asper a preferred working example of the invention for the usual threewashing stages of a wash of nitroaromatics;

FIG. 4 provides a schematic depiction of a production plant according tothe invention for nitrating nitratable aromatic compounds withsubsequent washing of the resulting nitroaromatics as per a preferredworking example of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention thus provides, according to a first aspect of thepresent invention, a process for removing impurities from crude nitratedproducts obtained in the nitration of nitratable aromatic compounds bytreating the crude product, after removal of the residual nitratingacid, with a washing medium, wherein the process of the inventioncomprises the following process steps:

-   -   in a first process step (a), the crude nitrated products are        firstly brought into contact with a washing medium and the crude        nitrated products and the washing medium are dispersed in one        another in such a way that a dispersion, in particular an        emulsion, results (i.e. in other words, a dispersion or emulsion        is produced from crude nitrated products and washing medium in        this first process step (a)) and    -   in a second process step (b), the resulting dispersion, in        particular emulsion, is subsequently fed into a tube reactor so        that the impurities initially present in the crude nitrated        products are removed during passage of the emulsion through the        tube reactor and/or so that the impurities initially present in        the crude nitrated products are transferred into the washing        medium or neutralized thereby during passage of the emulsion        through the tube reactor.

The process of the invention is thus outstandingly suitable forpurifying crude nitrated products obtained in the nitration ofnitratable aromatic compounds after removal of the residual nitratingacid.

The principle of the process of the invention therefore consists offirstly bringing the crude nitroaromatics originating from thenitration, which still contain significant amounts of impurities, intocontact, after removal of the residual nitrating acid (e.g. in aseparator), with a washing medium and converting the mixture ofnitroaromatics to be purified and washing medium into an emulsion ordispersion and subsequently feeding the resulting emulsion or dispersioninto a tube reactor so that the impurities initially present in thenitroaromatics to be purified are transferred into the washing medium orneutralized thereby, forming a purified nitroaromatic.

As the applicant has discovered, completely surprisingly, the use of atube reactor in combination with an upstream dispersing or emulsifyingdevice leads to particularly good mixing and particularly intimate andfine dispersion of washing medium and nitroaromatic to be purified beingable to be achieved, so that the impurities can in this way be removedcompletely or at least essentially completely in a single process step(namely in the treatment in the tube reactor).

In contrast to the prior art, further, complex process steps forpurifying the crude nitroaromatic are in this way avoided in anefficient manner without a reduction in quality having to be accepted inthe purification of the crude nitroaromatic.

The tube reactor used according to the invention for the treatment ofthe crude nitroaromatic with the washing medium surprisingly ensuressuch intimate and fine dispersion of the crude nitroaromatic and thewashing medium that all or at least essentially all impurities aretransferred into the washing medium or neutralized thereby in thetreatment in the tube reactor as per process step (b), so that theimpurities can subsequently (i.e. after conclusion of process step (b))be separated off together with the washing medium from the then purifiednitroaromatic.

It has surprisingly been found that it is possible in the context of thepresent invention to carry out washing of nitroaromatics successfully ineffect in a single stage, even in the case of high loading withimpurities such as nitrating acid, nitrophenols and nitrocresols, bymeans of a simple and inexpensive combination of jet mixers but alsoother dispersing devices, e.g. centrifugal pumps, with additionaldevices such as static mixers, orifice plates, etc., in tube reactorseither alone or in combination with stirred vessels which allow aprecisely defined mixing energy to be introduced into the mixture of themutually immiscible phases. The emulsions of the organic phase to bepurified in the washing medium (0/W type) or of the washing medium inthe organic phase (W/O type) which can be produced in this way producethe interface between nitroaromatic to be washed and washing mediumrequired for effective and optimal mass transfer.

As regards the production of the emulsion or dispersion in process step(a), this is generally carried out by means of a suitable dispersing oremulsifying device, in particular by means of a suitable mixing device.

In the context of the present invention, it is possible to use, forexample, a stirred vessel, a jet mixer (or Jet Mixing Device) or a pump,in particular a centrifugal pump, as dispersing or emulsifying device(i.e. in particular as preferably first dispersing or emulsifyingdevice), in particular as mixing device.

In an embodiment of the invention, a pump, in particular a centrifugalpump, is used as dispersing or emulsifying device, in particular asmixing device, in process step (a).

In an alternative embodiment which is preferred according to theinvention, a jet mixer (synonymously also referred to as “Jet MixingDevice”) is used as dispersing or emulsifying device, in particular asmixing device, in process step (a). The jet mixer used according to theinvention is, in particular, a device which produces a (central) drivingjet in a medium (e.g. annular jet) surrounding the (central) drivingjet.

As jet mixers, it is possible to use all types of jet mixers which allowthe nitroaromatic to be washed or the washing medium to be injected athigh relative velocity by means of the central driving jet as free jet,which can consist either of the washing medium or of the nitroaromaticto be washed, in such a way that either the nitroaromatic to be washedis dispersed as emulsion having a large interfacial area in the washingmedium or the washing medium is similarly dispersed in the nitroaromaticto be washed. Devices of this type are described, for example, inUllmann's Encyclopedia of Industrial Chemistry, 2003, 5th ed., Vol. B 4,pages 87/88 and 565 to 571, or else in Perry's Chemical Engineers'Handbook, McGraw-Hill Book Company, 1984, 6^(th) edition, pages 5-21 to5-23 or else in the German first publication DE 2 151 206.

Here, the (central) driving jet in the jet mixer can be the washingmedium and the surrounding medium can be the crude nitrated aromatic tobe purified; as an alternative, however, the (central) driving jet canalso be formed by the crude nitrated product to be purified and themedium surrounding the (central) driving jet can be formed by thewashing medium. Both alternative embodiments lead to the desired result.

Particularly good results in respect of the purification of the crudearomatic to be purified are (independently of whether the centraldriving jet is formed by the washing medium or else by the crudenitrated product to be purified) obtained when the ratio of thevelocities of the central driving jet and the medium (e.g. annular jet)surrounding the central driving jet in the jet mixer is set in the rangefrom 1:5 to 30:1, preferably in the range from 1:2 to 20:1, particularlypreferably in the range from 1:1 to 10:1. Particularly intimate and finedispersion of washing medium and crude product and consequentlyparticularly efficient purification are achieved in this way.

The flow velocity of the washing emulsion downstream of the jet mixer inthe subsequent tube reactor is, in particular, in the range from 0.1 to15.0 m/s, preferably in the range from 0.5 to 10 m/s.

According to an embodiment of the present invention, the dispersingdevice, in particular the mixing device, used in process step (a) can beinstalled upstream, in particular directly upstream, of the tubereactor. According to a particular variant of this embodiment, thedispersing or emulsifying device, in particular the mixing device, cango over into the tube reactor.

However, it is equally possible for the dispersing device, in particularthe mixing device, to be integrated into the tube reactor or be aconstituent of the tube reactor. For this purpose, the dispersing devicecan, for example, be arranged in the upper or upstream part of the tubereactor. Such an embodiment is particularly useful when the dispersingdevice, in particular the mixing device, is configured as a jet mixer.

In an embodiment which is particularly preferred according to theinvention, the tube reactor for carrying out process step (b) can beequipped with mixing elements, in particular for the introduction ofadditional mixing energy; particularly good purification results can beachieved in this way since a further improved, particularly intimatedispersion of washing medium and crude aromatic to be purified isachieved by means of the additional mixing elements. The mixing elementscan be, in particular, metal plates, in particular impingement plates ordeflection plates, orifice plates, static mixers, flow dividers or thelike. According to the invention, it is preferable for from 1 to 15, inparticular from 2 to 15, preferably from 2 to 10, particularlypreferably from 2 to 5, mixing elements to be present in the tubereactor.

In this embodiment, preference is given to the mixing elements providedin the tube reactor introducing a total mixing energy (i.e. a mixingenergy per unit volume) of from 10 to 1000 joule/liter, preferably from10 to 500 joule/liter, particularly preferably from 20 to 200joule/liter. In other words, a total mixing energy (i.e. a mixing energyper unit volume) of from 10 to 1000 joule/liter, preferably from 10 to500 joule/liter, particularly preferably from 20 to 200 joule/liter, ispreferably introduced in this embodiment.

Particularly good results are, in particular, also achieved when themixing elements are configured so that the pressure drop per mixingelement is from 0.1 bar to 3.0 bar, preferably from 0.3 to 1.5 bar,particularly preferably from 0.3 to 0.8 bar.

As regards the residence time of the emulsion of washing medium andcrude aromatics in the tube reactor in process step (b), this can varywithin a wide range. Particular preference is given to the residencetime in the tube reactor being from 0.1 to 120 seconds, preferably from0.1 to 60 seconds, particularly preferably from 1 to 30 seconds.Particularly good washing results are achieved in this way since asufficient minimum residence time but also an economic throughput areensured.

In the purification, the mass ratio and phase ratio of crude nitratedproducts to be purified to washing medium are also important and caneach vary within a wide range.

Particularly good results are obtained when the mass ratio of crudenitrated products to be purified to washing medium (i.e. freshlyintroduced washing medium) is set in the range from 200:1 to 1:10,preferably in the range from 100:1 to 1:5, particularly preferably inthe range from 10:1 to 1:2.

Particularly good results are equally obtained when the phase ratio(i.e. in particular the phase ratio in the washing apparatus) of crudenitrated products to be purified to washing medium is set in the rangefrom 25:1 to 1:5, in particular in the range from 10:1 to 1:2,preferably in the range from 5:1 to 1:1. The setting of the phase ratiocan, in particular, be effected by circulation of the washing mediumafter phase separation. This ensures firstly an optimal exchange areabetween organic phase and washing medium and secondly a very short timefor the phase separation in the phase separation apparatus.

The washing of the nitroaromatics is usually carried out as aliquid/liquid wash (i.e. at temperatures at which both the nitroaromaticto be washed and purified and also the washing medium are present asliquids).

As regards the washing medium used according to the invention, this isliquid under process conditions, in particular at temperatures at orabove 5° C., in particular at temperatures at or above 25° C., andatmospheric pressure. According to the invention, preference is given tousing an aqueous-based washing medium, preferably water.

Depending on the phase ratio in the washing apparatus, the nitroaromaticto be washed is dispersed in the washing medium as an oil-in-wateremulsion (O/W emulsion) or the washing medium is dispersed in thearomatic to be washed as a water-in-oil emulsion (W/O emulsion).

The efficiency of the washing medium can be increased by adding at leastone base to the washing medium. The base can, in particular, be selectedfrom the group consisting of inorganic hydroxides, carbonates,hydrogencarbonates, sulfites, hydrogen sulfites and ammonia and alsomixtures or combinations thereof, preferably from the group consistingof sodium hydroxide, potassium hydroxide, sodium carbonate, potassiumcarbonate, sodium hydrogencarbonate, potassium hydrogencarbonate,ammonia, ammonium carbonate, sodium sulfite and sodium hydrogen sulfiteand also mixtures or combinations thereof.

The amount of alkali used in an alkaline wash should, in particular, besufficiently high not only for all acids to be able to be convertedquantitatively into their salts but, in particular, an excess of baseshould be used so that the pH of the washing liquor is sufficiently highfor even weak acids such as mononitrophenols to be able to be washed outquantitatively.

The content of alkali can be, in particular, from 0.01 mol/l to 0.4mol/l, preferably from 0.02 mol/l to 0.2 mol/l, but at least twice theamount required for neutralization of all nitrophenols.

Particularly good results are obtained when the content of base in thewashing medium is from 0.01 to 0.4 mol/l, preferably from 0.02 to 0.2mol/l.

In particular, the content of base in the washing medium should be atleast twice the amount of alkali which is theoretically required forneutralization of all nitrophenols present as impurities.

As indicated above, the phase ratio of nitroaromatic to be washed tofreshly introduced washing medium should advantageously be from 200:1 to1:10, preferably from 100:1 to 1:5, particularly preferably from 10:1 to1:2. Circulation of the washing medium after phase separation makes itpossible to set a phase ratio of nitroaromatic to be washed to washingmedium in the washing apparatus of from 25:1 to 1:5, in particular from10:1 to 1:2, particularly preferably from 5:1 to 1:1, in order firstlyto produce an optimal exchange area between organic phase and washingmedium and secondly to keep the time for the phase separation in thephase separation apparatus as short as possible.

Depending on the phase ratio in the washing apparatus, the nitroaromaticto be washed is dispersed in the washing medium as an oil-in-wateremulsion (O/W emulsion) or the washing medium is dispersed in thearomatic to be washed as a water-in-oil emulsion (W/O emulsion) (cf.what has been said above).

Depending on the phase ratio selected, either the aromatic to be washedor the washing medium is used as driving jet in order to set the desiredtype of emulsion.

The flow velocity of the washing emulsion downstream of the jet mixer inthe subsequent tube reactor can be, in particular, in the range from 0.1to 15.0 m/s, preferably from 0.5 to 10 m/s.

The ratio of the velocity of the central jet to that of the surroundingmedium is, as indicated above, in the range from 1:5 to 30:1, preferablyfrom 1:2 to 20:1 and particularly preferably from 1:1 to 10:1.

To prevent the coalescence of the washing emulsion after a short timeand thus prevent incomplete extraction of the impurity to be removedfrom the nitroaromatic to be purified, it is advantageous to keep thewashing emulsion stable by additional introduction of mixing energyuntil all impurities have been washed from the nitroaromatic andre-extraction into the nitroaromatic to be washed is prevented byfurther reactions in the washing medium. This additional mixing energycan be introduced into the mixture of the two mutually immiscible phasesby feeding into a reactor having additional mixing devices, preferablyinto a tube reactor without backmixing, with the emulsion of the O/W orW/O type being maintained in the tube reactor by means of additionalmixing elements distributed over the tube reactor, e.g. orifice plates,deflection plates, baffles, static mixers or other static mixingelements.

Preference is given to from 1 to 15, in particular from 2 to 15,preferably from 2 to 10 and particularly preferably from 2 to 5, mixingelements being present in the tube reactor, with the jet mixer alsocounting as mixing element.

The total mixing energy per unit volume to be introduced should be from10 to 1000 J/l, preferably from 10 to 500 J/l and particularlypreferably from 20 to 200 J/l.

The pressure drop per mixing element should be from 0.1 to 3.0 bar,preferably from 0.2 to 1.5 bar and particularly preferably from 0.2 to0.8 bar, in order to keep the number of additional mixing elementsrequired in the tube reactor as low as possible and to keep theresidence time in the phase separation apparatus as short as possible.

The residence time in the tube reactor for separating off acids,followed by a fast further reaction such as neutralization, from thenitroaromatic to be washed, e.g. nitric acid, sulfuric acid,mononitrophenols, dinitrophenols and trinitrophenols and cresols,nitrobenzoic acids, etc., in a wash by means of alkali, e.g. sodiumhydroxide, sodium carbonate, bicarbonate, ammonia, potassium hydroxide,etc., should be not more than from 0.1 to 120 seconds, preferably from0.1 to 60 seconds, particularly preferably from 1 to 30 seconds.

To remove impurities from the nitroaromatic to be washed with highpartition coefficients in favor of the nitroaromatic to be washed, highmass transfer resistances in the organic phase and slow furtherreactions of the extracted impurity in the washing medium, e.g. nitrogendioxide, the residence time in the subsequent reactor should be matchedto these circumstances (e.g. by a combination of the above-describeddevices for producing an optimal washing emulsion with stirred vesselsin order to produce the necessary residence time). In a particularembodiment of the process of the invention, this is, in particular,achieved by a combination of the above-described devices for producingan optimal washing emulsion with stirred vessels in order to ensure thenecessary residence time for phase transfer and the subsequent reaction.

As indicated above, the amount of alkali used in an alkaline wash shouldnot only be sufficiently high for all acids to be able to be convertedquantitatively into their salts but an excess of base should also beused so that the pH of the washing liquor is sufficiently high for evenweak acids such as mononitrophenols to be able to be washed outquantitatively. As indicated above, the content of alkali should be, inparticular, from 0.01 mol/l to 0.4 mol/l, preferably from 0.02 mol/l to0.2 mol/l, but at least twice the amount required for neutralization ofall nitrophenols.

The emulsion present at the end of the mixing section can, for example,be separated again into the individual phases in a phase separationapparatus (e.g. separator or settler). The washing medium with theimpurities present therein can either be passed as wastewater to awastewater treatment or can be introduced in countercurrent into thepreceding washing stage.

The washed nitroaromatic can either be fed into the subsequent washingstage or at the end of the wash be transferred directly to furtherprocessing or to intermediate storage.

As phase separation apparatus, it is possible to use all types of staticseparators but also dynamic separators such as centrifugal separators.The separation time for the nitroaromatic/washing medium emulsiondepends not only on the type of emulsion (W/O or O/W) and the mixingenergy introduced but also on the excess of base in the washing mediumwhich is not required for neutralization. With introduction of the samemixing energy, the separation time decreases significantly withincreasing base concentration in the washing medium. However,surface-active agents or mechanical separation aids, e.g. packings,separation plates, etc., can also be used to accelerate phaseseparation. The phase separation can also be accelerated by a spacingbetween the individual mixing elements which is matched to thenitroaromatic and type of emulsion.

As regards the crude nitrated products to be purified, these aregenerally liquid under process conditions, in particular at temperaturesat or above 5° C., in particular at temperatures at or above 25° C., andatmospheric pressure. In particular, the crude nitrated products to bepurified originate from the nitration of monocyclic or polycyclicaromatics, in particular from the nitration of benzene, toluene, xyleneor halogenated aromatics such as, in particular, chlorinated benzenes.

The crude nitrated products to be purified are, in particular,optionally halogenated mononitroaromatics, dinitroaromatics andtrinitroaromatics, e.g. nitrobenzene (MNB), mononitrotoluene (MNT),dinitrotoluene (DNT), trinitrotoluene (TNT), nitrochlorobenzene (MNCB)or the like.

In general, the process step (b) is followed by separation of thenitrated products which have been freed of the impurities from thewashing medium. This separation is generally carried out by means of asuitable separation apparatus (separator or settler).

Furthermore, in a particular embodiment of the process of the invention,the mixture of purified nitrated products and washing medium leaving thetube reactor, in particular before the nitrated products which have beenfreed of the impurities have been separated off from the washing medium,can firstly be transferred into a stirred vessel. In this way, thecontact and/or residence time between nitration products to be purifiedand washing medium is efficiently increased so that impurities whichhave not yet been washed out may be transferred into the washing mediumor neutralized thereby.

In an advantageous embodiment of the process of the invention, thewashing medium, in particular after the nitrated products which havebeen freed of the impurities have been separated off from the washingmedium, is recycled. This makes efficient washing or circulationpossible and reduces the amount of washing medium to a minimum.

Any residual amounts or traces of water still present, in particularsuspended and/or dissolved water, can optionally be removed by dryingfrom the purified nitroaromatic after the wash or after the washingmedium has been separated off (e.g. after separation of the washingemulsion in a static separator or by means of a centrifugal separator).

The process of the invention is suitable for carrying out the acid washand/or the basic wash and/or the neutral wash of the crude nitratedproducts. The process of the invention can thus be used in all three ofthe abovementioned washing steps. However, it is equally possible to usethe process of the invention for only one or two washing stages, forexample only for the acid wash or else only for the basic wash or elseonly for the neutral wash. In this respect, the process of the inventioncan be used flexibly.

As indicated above, the process of the invention is associated with anumber of advantages and particular aspects, of which the following fewadvantages and particular aspects will be mentioned, but notexhaustively and not implying any restriction:

In particular, the process of the invention allows efficientpurification of crude nitrated products obtained in the nitration ofnitratable aromatic compounds after removal of the residual nitratingacid, with only a low complexity and good process economics, e.g.process efficiency.

The tube reactor used according to the invention makes efficient andintimate dispersion of washing medium and crude nitrated aromatics inone another possible, so that no further washing steps or othertreatment steps are required. The washing or treatment efficiency can beincreased further by additional mixing elements which, as indicatedabove, further improve mixing being provided in the tube reactor.

The tube reactor employed according to the invention for thepurification can equally well be used as reaction vessel in thepreceding nitration, so that no additional apparatus has to be used forthe purification of the crude nitrated products.

The tube reactor employed according to the invention for thepurification of the crude nitration products makes it possible toproduce large exchange areas in a two-phase mixture of washing mediumand crude nitrated aromatics, so that effective mass transfer and rapidtransfer of the impurities into the washing medium, or in the case ofacidic compounds rapid neutralization, are ensured in this way.

Furthermore, the process of the invention makes it possible for theimpurities coming from the nitration to be removed rapidly and at thesame time efficiently from the crude nitrated products, with the washingmedium being able to be readily recycled or circulated after thetreatment of the crude nitrated aromatics.

The present invention further provides, according to a second aspect ofthe present invention, an apparatus (plant) for removing impurities fromcrude nitrated products obtained in the nitration of nitratable aromaticcompounds by treating the crude products, after removal of the residualnitrating acid, with a washing medium, where the apparatus of theinvention is particularly suitable for carrying out a process asdescribed above,

wherein the apparatus has the following devices:

-   (a) at least one dispersing device, in particular at least one    mixing device, for contacting and emulsifying crude nitrated    products to be purified and washing medium; and,-   (b) arranged downstream of the dispersing device, a tube reactor for    introduction of the emulsion of crude nitrated products to be    purified and washing medium produced in the dispersing device, where    the tube reactor is configured so that removal of the impurities    initially present in the crude nitrated products is made possible    during passage of the emulsion through the tube reactor and/or that    the impurities initially present in the crude nitrated products are    transferred into the washing medium and/or neutralized thereby    during passage of the emulsion through the tube reactor.

As indicated above in connection with the process of the invention, thedispersing device, in particular the mixing device, can be a stirredvessel, a jet mixer or a pump, in particular a centrifugal pump,preferably a pump, in particular a centrifugal pump, or a jet mixer,particularly preferably a jet mixer.

As indicated above in the context of the process of the invention, thedispersing device, in particular the mixing device, can be installedupstream of the reactor, in particular directly upstream. In particular,the dispersing device, in particular the mixing device, can in this casego over into the tube reactor.

In an alternative embodiment, the dispersing device, in particular themixing device, can be integrated into the tube reactor and/or be aconstituent of the tube reactor. In this regard, reference may be madeto what has been said above in connection with the process of theinvention.

As explained above in the description of the process of the invention,the tube reactor can be equipped with mixing elements, in particular forintroduction of additional mixing energy. As described above, the mixingelements can be configured as plates, in particular impingement platesor deflection plates, as orifice plates, as static mixers or as flowdividers.

A single-stage, two-stage or three-stage wash of the crude nitrationproduct (e.g. acid wash and/or basic wash and/or neutral wash) can becarried out in the apparatus of the invention.

Furthermore, it is possible, according to the invention, for aseparation device, in particular a separator or settler and/or dynamicseparator or centrifugal separator, to be arranged downstream of thetube reactor in order to separate off the nitrated products which havebeen freed of the impurities from the washing medium.

In addition, it is possible, in the apparatus of the invention, for astirred vessel and/or stirred reactor to be arranged downstream of thetube reactor and upstream of the separation device (i.e. in other wordsbetween tube reactor and separation device). In particular, the contactand/or residence time between nitrated products and the washing mediumis increased in this way.

For further details regarding the apparatus or plant of the invention,reference may be made to what has been said above in respect of theprocess of the invention, which applies analogously to the apparatus orplant of the invention.

Finally, the present invention further provides, according to a thirdaspect of the present invention, a production plant for nitratingnitratable aromatic compounds with subsequent purification of the crudenitrated products formed in the nitration,

wherein the production plant comprises the following units:

-   (a) a nitration unit for nitrating aromatic compounds, in particular    having one or more appropriate reaction vessels for carrying out the    nitration reaction(s);-   (b) optionally, arranged in the production line downstream of the    nitration unit, at least one separation device, in particular a    separator, for separating off the residual nitrating acid from the    crude nitrated products;-   (c) arranged in the production line downstream of the nitration unit    and any separation device present, at least one washing device for    carrying out washing of the crude nitrated products, where the    washing device comprises:    -   at least one dispersing device, in particular at least one        mixing device, for contacting and emulsifying the crude nitrated        products to be purified and the washing medium and,    -   arranged downstream of the dispersing device, a tube reactor for        introduction of the emulsion of crude nitrated products to be        purified and washing medium produced in the dispersing device,        where the tube reactor is configured so that removal of the        impurities initially present in the crude nitrated products is        made possible during passage of the emulsion through the tube        reactor and/or that the impurities initially present in the        crude nitrated products are transferred into the washing medium        and/or neutralized thereby during passage of the emulsion        through the tube reactor;-   (d) optionally, arranged in the production line downstream of the    washing device, a stirred vessel, in particular for increasing the    contact and/or residence time between nitrated products and washing    medium;-   (e) arranged in the production line downstream of the washing unit    and any stirred vessel present, a separation device, in particular a    separator, for separating off the nitrated products which have been    freed of the impurities from the washing medium.

In other words, in the production plant of the invention, theabove-described apparatus or plant for purification, i.e. for removal ofimpurities, is a constituent of this production plant, namely in theform of the washing unit or washing device (c).

As indicated above, the dispersing device, in particular the mixingdevice, can be a stirred vessel, a jet mixer or a pump, in particular acentrifugal pump, preferably a pump, in particular a centrifugal pump,or a jet mixer, particularly preferably a jet mixer, in the productionplant of the invention, too.

In a particular embodiment of the production plant of the invention, thedispersing device, in particular the mixing device, can, as indicatedabove, be installed upstream of the reactor, in particular directlyupstream of the reactor. In this embodiment, the dispersing device, inparticular the mixing device, can, in particular, go over into the tubereactor.

It is likewise possible according to the invention for the dispersingdevice, in particular the mixing device, to be integrated into the tubereactor and/or to be a constituent of the tube reactor. As regards thisembodiment, reference may be made to what has been said above in orderto avoid unnecessary repetition.

As indicated above in connection with the process of the invention andin connection with the purification apparatus or plant of the invention,the tube reactor can be equipped with mixing elements, in particular forintroduction of additional mixing energy. In this embodiment, the mixingelements can, in particular, be configured as plates, in particularimpingement plates or deflection plates, as orifice plates, as staticmixers or as flow dividers.

The process of the invention is particularly suitable for carrying outan acid wash and/or a basic wash and/or a neutral wash of crude nitratedproducts. The process of the invention can thus be employed in all threeabovementioned washing steps of a washing device. However, it is equallypossible to use the process of the invention for only one or two washingsteps, for example only for an acid wash or else only for a basic washor else only for a neutral wash. In this respect, the process of theinvention can be used flexibly.

For further details regarding the production plant of the invention,reference may be made to what has been said above in respect of theprocess of the invention and the apparatus or plant of the invention,with this applying analogously to the production plant of the invention.

The process of the invention and the apparatus or plant of the inventionfor purification and also the production plant according to theinvention for nitration are illustrated by way of example and in anonrestrictive manner in the attached figures.

Further advantages, properties, aspects and features of the presentinvention can be seen from the following description of embodimentswhich are preferred according to the invention and are shown in thedrawings. The figures show:

FIG. 1 a schematic depiction of a wash of nitroaromatics according tothe prior art by means of mixer/settler technology for the usual threewashing stages of a wash of nitroaromatics;

FIG. 2 a schematic depiction of a single-stage wash for nitroaromaticsaccording to the process of the invention or using the apparatus orplant of the invention;

FIG. 3 a schematic flow diagram of the process of the invention or aschematic depiction of the apparatus or plant of the invention as per apreferred working example of the invention for the usual three washingstages of a wash of nitroaromatics;

FIG. 4 a schematic depiction of a production plant according to theinvention for nitrating nitratable aromatic compounds with subsequentwashing of the resulting nitroaromatics as per a preferred workingexample of the invention.

FIG. 1 shows an example of a wash of nitroaromatics in three stepsaccording to the prior art:

-   a) In step 1, the sulfuric acid and nitric acid suspended and    dissolved in the crude nitroaromatic (NA 10) are washed out by    washing with fresh water (WW 10) in a multistage continuous acid    wash (WS). The nitroaromatic (NA 10) to be washed and the washing    water (WW 10) are fed into a mixing device, usually a stirred    vessel, having a residence time of about 10 minutes. The washing    emulsion formed is subsequently separated in a separator (S). To    completely remove the dissolved and suspended mineral acids, it is    possible to use up to 4 mixer/settler units (n=3), with the washing    medium and the nitroaromatic to be washed being conveyed in    countercurrent. The washing medium is, after phase separation,    either all discharged immediately as wastewater (WW 11) or part of    it is additionally circulated in order to set a prescribed phase    ratio and thus a defined type of emulsion and to minimize the time    for separation of the phases. The nitroaromatic (NA 11) which has    been freed of mineral acids is fed into washing stage 2, the    alkaline wash (WA).-   b) In step 2, all dissolved nitrophenols, nitrobenzoic acids and    other acidic materials from the oxidative degradation of impurities    and isomeric nitroaromatics are removed from the nitroaromatic (e.g.    TNT) in a multistage continuous alkaline wash (WA). The    nitroaromatic (NA 11) to be washed and the washing water (WW 10 or    WW 13) are, together with a base, fed into a stirred vessel having a    residence time of about 10 minutes. The washing emulsion formed is    subsequently separated in a separator (S). To completely remove the    nitrophenols, nitrobenzoic acids and other acidic materials from the    oxidative degradation of impurities and isomeric nitroaromatics    which are dissolved in the nitroaromatic, it is possible to use up    to 4 mixer/settler units (n=3), with the washing medium and the    nitroaromatic to be washed being conveyed in countercurrent. The    washing medium is, after phase separation, either all discharged    immediately as wastewater (WW 12) or part of it is additionally    circulated in order to set a prescribed phase ratio and thus a    defined type of emulsion and to minimize the time for separation of    the phases. The nitroaromatic (NA 12) which has been freed of    mineral acids, nitrophenols, nitrobenzoic acids and other acidic    materials from the oxidative degradation of impurities and isomeric    nitroaromatics is fed into washing stage 3, the neutral wash (WN).-   c) In step 3, the entrained traces of washing medium from the    alkaline wash (WA) are removed in a multistage neutral wash (WN).    The nitroaromatic (NA 12) to be washed and the washing water (WW 10)    are fed into a stirred vessel having a residence time of about 10    minutes. The washing emulsion formed is subsequently separated in a    separator (S). To completely remove the traces of base still    suspended or dissolved in the nitroaromatic, it is possible to use    up to 4 mixer/settler units (n=3), with the washing medium and the    nitroaromatic to be washed being conveyed in countercurrent. The    aqueous phase is either all fed immediately as washing medium (WW    13) into the alkaline wash (WA) or part of it is additionally    circulated in order to set a prescribed phase ratio and thus a    defined type of emulsion and to minimize the time for separation of    the phases.-    The nitroaromatic (NA 13) which has now been freed of mineral    acids, nitrophenols, nitrobenzoic acids and other acidic materials    from the oxidative degradation of impurities, isomeric    nitroaromatics and residual traces of alkali is passed on directly    for further processing or to intermediate storage.

FIG. 2 shows an embodiment for one washing stage according to theprocess of the invention or in the apparatus or plant of the inventionfor washing nitroaromatics using the washing medium as driving jet.

The nitroaromatic to be washed is, after removal of the residualnitrating acid (NA1 (n−1) where n=1) or after removal of the residualnitrating acid still suspended as microemulsion in the nitroaromatic orof the sulfuric acid, nitric acid and nitrogen dioxide still dissolvedin the nitroaromatic in an acid wash (WS with n=2) or after removal ofall nitrophenols, nitrobenzoic acids and other acidic materials from theoxidative degradation of impurities and isomeric nitroaromaticsdissolved in the nitroaromatic from the nitroaromatic (e.g. TNT) in thepresence of bases in an alkaline wash (WA with n=3), combined in a jetmixer (SM) with the washing medium WW1 (n−1) which in the case shownserves as driving jet and introduced directly into a tube reactor (C)which contains additional mixing elements (Mm).

To set an increased residence time so as to permit slow reactions of theimpurities to be washed out in the washing medium, e.g. nitrogendioxide, the washing emulsion can be fed from the tube reactor into aresidence vessel, e.g. one or more stirred vessels (R). The washingemulsion from the tube reactor is separated either directly or after aprolonged residence time in the stirred vessel into the phases in aseparation device.

The washed nitroaromatic (NA1 n where n=1 to 3) is discharged eitherinto the subsequent washing stage or as a finished washed product (NA13)for further processing. The loaded washing medium (WW1 n where n=1 to 3)is either discharged directly as wastewater or recirculated as substreamto set a defined phase ratio between nitroaromatic and washing medium.This recirculated substream can be fed either together with the freshlyadded washing water as driving jet or as circulating stream directlyinto the tube reactor.

FIG. 3 shows an example of the process of the invention in three stepsfor the separate removal of the mineral acids by means of acid wash(WS), the removal of all dissolved nitrophenols, nitrobenzoic acids andother acidic materials from the oxidative degradation of impurities andisomeric nitroaromatics in the presence of bases in the alkaline rangeby means of an alkaline wash (WA) and a neutral wash (WN).

-   a) In step 1, the sulfuric acid and nitric acid suspended and    dissolved in the crude nitroaromatic (NA 10) are removed by washing    with fresh water (WW 10) in a single-stage acid wash (WS). The    nitroaromatic (NA 10) to be washed and the washing water (WW 10) are    fed by means of pumps (P) either via a jet mixer or directly into a    tube reactor containing additional mixing elements (Mn). After    passing through the tube reactor, the emulsion formed is separated    in a separator (S). The washing medium is, after phase separation,    either discharged directly as wastewater (WW 11) or part of it is    additionally circulated for setting a prescribed phase ratio and    thus a defined type of emulsion and to minimize the time for    separation of the phases. The nitroaromatic (NA 11) which has been    freed of mineral acids is fed into washing stage 2, the alkaline    wash (WA).-   b) In step 2, all dissolved nitrophenols, nitrobenzoic acids and    other acidic materials from the oxidative degradation of impurities    and isomeric nitroaromatics are removed from the nitroaromatic in a    single-stage alkaline wash (WA). The nitroaromatic (NA 11) to be    washed coming from the acid wash (WS) and the washing water (WW 10    or WW 13 from the neutral wash) and a base are fed by means of    pumps (P) either via a jet mixer or directly into a tube reactor    containing additional mixing elements (Mn). After passing through    the tube reactor, the emulsion formed is separated in a separator.    The washing medium, which contains all dissolved nitrophenols,    nitrobenzoic acids and other acidic materials from the oxidative    degradation of impurities and extracted isomeric nitroaromatics    (dissolved as salt), is, after phase separation, either discharged    directly as wastewater (WW 12) or part of it is circulated to set a    prescribed phase ratio and thus a defined type of emulsion and to    minimize the time for separation of the phases. The nitroaromatic    (NA 12) which has been freed of mineral acids, nitrophenols,    nitrobenzoic acids and other acidic materials from the oxidative    degradation of impurities and isomeric nitroaromatics is fed into    washing stage 3, the neutral wash (WN).-   c) In step 3, the entrained traces of washing medium from the    alkaline wash are removed in a single-stage neutral wash (WN). The    nitroaromatic (NA 12) to be washed and the washing water (WW 10) are    fed by means of pumps (P) either via a jet mixer or directly into a    tube reactor containing additional mixing elements (Mn). After    passing through the tube reactor, the emulsion formed is separated    in a separator (S). The washing medium, which contains the residual    traces of alkali and impurities, is either introduced directly as    wastewater (WW 13) into the washing stage 2 (WA) or part of it is    additionally circulated to set a prescribed phase ratio and thus a    defined type of emulsion and to minimize the time for separation of    the phases. The nitroaromatic (NA 13) which has now been freed of    mineral acids, nitrophenols, nitrobenzoic acids and other acidic    materials from the oxidative degradation of impurities, isomeric    nitroaromatics and residual traces of alkali is passed directly to    further processing or to intermediate storage.

FIG. 4 shows an example of a production plant for preparingnitroaromatics which has an integrated wash according to the inventionof the crude nitroaromatics from an isothermal or adiabatic nitration.The crude nitroaromatic (NA 10) formed in the nitration unit (N) byreaction of the aromatic with nitric acid in the presence of sulfuricacid is, after removal of the nitrating acid in the separator (S),washed with water (WW 10) in the manner according to the invention inthe acid wash (WS). After phase separation, the resulting wastewater (WW11), which contains all washed-out sulfuric acid and nitric acid, isrecirculated together with the nitric acid (WNA) obtained from theoffgas treatment of the nitration plant in an absorber plant (A), eitherdirectly or after concentration in an SAC plant (SAC), together with theresidual acid (AS) from the nitration back to the nitration ordischarged as wastewater to be treated.

The nitroaromatic (NA 11) which has been freed of the mineral acids iswashed in effect in one stage in the presence of bases by the process ofthe invention in the washing stage 2 (alkaline wash WA). After phaseseparation, the wastewater from the alkaline wash (WW12), which has a pHin the range from 8.0 to 13 and contains all nitrophenols, nitrobenzoicacids and other acidic materials from the oxidative degradation ofimpurities and isomeric nitroaromatics (e.g. TNT), is fed to anadditional treatment, e.g. a thermolysis, before discharge into a mainoutfall.

The nitroaromatic (NA 12) from the alkaline wash (WA) is fed into theneutral wash (WN) and washed with water (WW 10) in effect in one stageby the process of the invention. After phase separation, the wastewater(WW 13) from the neutral wash (WN) is fed together with base into thewashing stage 2 (WA). The washed nitroaromatic (NA 13) is passed tofurther processing, e.g. to isomer separation or reduction to thecorresponding amine, or to intermediate storage.

Further embodiments, modifications and variations of the presentinvention can be readily recognized and realized by a person skilled inthe art on reading the description, without going outside the scope ofthe present invention.

The present invention is illustrated with the aid of the followingworking examples, but without the present invention being restrictedthereto.

Even though, in the following working examples, the process of theinvention or the apparatus of the invention is illustrated usingnitrobenzene as nitroaromatic to be purified, the process or theapparatus of the present invention is in no way restricted thereto butcan also be applied to any other nitroaromatics, e.g. from the nitrationof toluene, chlorobenzenes, xylenes, nitrobenzenes, etc., and to anybases other than sodium hydroxide.

WORKING EXAMPLES Example 1 Single-Stage Alkaline Wash (ComparativeExample)

12 kg/h of a nitrobenzene from an adiabatic nitration, which had beenprewashed with water (acid wash) and still contained a total of 1910 ppmof nitrophenols (0.8 ppm of 2-nitrophenol (2-NP), 1346 ppm of2,4-dinitrophenol (2,4-DNP) and 203 ppm of 2,6-dinitrophenol (2,6-DNP)and 360 ppm of picric acid (2,4,6-TNP)), was fed together with a washingliquor containing 0.8 g of NaOH/l (two-fold excess, based on allnitrophenols) in a weight ratio of 1:1 into a stirred vessel at 60° C.The stirrer speed was set so that an O/W emulsion having the phase ratioas metered in was present in the stirred vessel. The residence time inthe stirred vessel was 6 minutes. After phase separation (about 40minutes), the pH of the washing liquor, which contained 1850 ppm ofnitrophenols, was about 11.7. 60 ppm of nitrophenols were found in thewashed nitrobenzene. When a washing liquor containing 4 g/l of sodiumhydroxide was used under otherwise identical conditions, the separationtime could be shortened by a factor of virtually 4 to about 15 minutes.

Example 2 Single-Stage Alkaline Wash (According to the Invention)

12 kg/h of a nitrobenzene from an adiabatic nitration, which had beenprewashed with water (acid wash) and still contained a total of 1910 ppmof nitrophenols (0.8 ppm of 2-nitrophenol (2-NP), 1346 ppm of2,4-dinitrophenol (2,4-DNP) and 203 ppm of 2,6-dinitrophenol (2,6-DNP)and 360 ppm of picric acid (2,4,6-TNP)), was fed together with a washingliquor containing 0.8 g of NaOH/l (two-fold excess, based on allnitrophenols) in a weight ratio of 1:1 by means of a jet mixer using thewashing medium as central jet at 60° C. into a tube reactor whichadditionally contained 5 static mixing elements. The relative velocitybetween central jet and nitrobenzene to be washed was 8:1. The residencetime in the tube reactor was not more than 5 seconds. The pressure dropover the entire length of the tube reactor was 1.6 bar. After phaseseparation of the O/W emulsion (about 40 minutes), the pH of the washingliquor, which contained 1908 ppm of nitrophenols, was about 11.6. 2 ppmof nitrophenols were found in the washed nitrobenzene. When a washingliquor containing 4 g/l of sodium hydroxide was used under otherwiseidentical conditions, the separation time could be shortened by a factorof 4 to about 10 minutes. The same results were achieved using thenitroaromatic to be washed as central jet in the jet mixer.

Example 3 Single-Stage Neutral Wash (According to the Invention)

12 kg/h of a nitrobenzene from an adiabatic nitration, which afterwashing with alkali (see, for example, example 2, alkaline wash) stillcontained a total of from 2 to 5 ppm of nitrophenols, was fed in aweight ratio of 1:1 by means of a jet mixer using water as central jetat 60° C. into a tube reactor which additionally contained 2 staticmixing elements. The relative velocity between central jet andnitrobenzene to be washed was 8:1. The residence time in the tubereactor was about 5 seconds. The pressure drop over the entire length ofthe tube reactor was 0.6 bar. After phase separation (about 25 minutes),the pH of the washing water containing from about 1.5 to 4.5 ppm ofnitrophenols was about 9.0. 0.5 ppm of nitrophenols was still found inthe washed nitrobenzene. The same results were achieved using thenitroaromatic to be washed as central jet in the jet mixer.

Example 4 Single-Stage Alkaline Wash (According to the Invention)

20 kg/h of a nitrobenzene from an adiabatic nitration, which had beenprewashed with water (acid wash) and still contained a total of 1910 ppmof nitrophenols (0.8 ppm of 2-nitrophenol (2-NP), 1346 ppm of2,4-dinitrophenol (2,4-DNP) and 203 ppm of 2,6-dinitrophenol (2,6-DNP)and 360 ppm of picric acid (2,4,6-TNP)), was washed directly with 4 kg/hof washing liquor containing 4 g of NaOH/l (two-fold excess, based onall nitrophenols), corresponding to a weight ratio of nitroaromatic towashing liquor of 5:1, with the washing medium being fed by means of ajet mixer and the nitroaromatic to be washed at 60° C. into a tubereactor which additionally contained 5 static mixing elements. Therelative velocity between central jet and nitrobenzene to be washed was8:1. The residence time in the tube reactor was not more than 5 seconds.The pressure drop over the entire length of the tube reactor was 1.6bar. After phase separation of the emulsion of the W/O type (about 5minutes), the pH of the washing liquor, which contained 9552 ppm ofnitrophenols, was about 12.3. About 8 ppm of nitrophenols were found inthe washed, still turbid nitrobenzene.

Example 5 Single-Stage Neutral Wash (According to the Invention)

20 kg/h of a nitrobenzene from an adiabatic nitration, which afterwashing with alkali (see example 2, alkaline wash) still contained atotal of from 5 to 8 ppm of nitrophenols, was fed in a weight ratio of5:1 by means of a jet mixer using water as central jet at 60° C. into atube reactor which additionally contained 2 static mixing elements. Therelative velocity between central jet and nitrobenzene to be washed was8:1. The residence time in the tube reactor was about 5 seconds. Thepressure drop over the entire length of the tube reactor was 0.6 bar.After phase separation (about 20 minutes), the pH of the washing watercontaining from about 1.5 to 4.5 ppm of nitrophenols was about 9.0. 0.5ppm of nitrophenols was still found in the washed nitrobenzene. The sameresults were achieved using the nitroaromatic to be washed as centraljet in the jet mixer.

1. A production plant for nitrating nitratable aromatic compounds withsubsequent purification of the crude nitrated products formed in thenitration, wherein the production plant comprises the following units:(a) a nitration unit for nitrating aromatic compounds, wherein thenitration unit comprises one or more appropriate reaction vessels forcarrying out the nitration reaction(s); (b) optionally, arranged in theproduction line downstream of the nitration unit, at least oneseparation device for separating off the residual nitrating acid fromthe crude nitrated products; (c) arranged in the production linedownstream of the nitration unit and the optional separation device ifpresent, at least one washing device for carrying out washing of thecrude nitrated products, where the washing device comprises: at leastone dispersing device for contacting and emulsifying the crude nitratedproducts to be purified and the washing medium, and, arranged downstreamof the dispersing device, a tube reactor for introduction of theemulsion comprising the crude nitrated products to be purified and thewashing medium and produced in the dispersing device, wherein the tubereactor is configured such that the impurities initially present in thecrude nitrated products are removed during passage of the emulsionthrough the tube reactor and/or such that the impurities initiallypresent in the crude nitrated products are transferred into the washingmedium and neutralized thereby during passage of the emulsion throughthe tube reactor; (d) optionally, arranged in the production linedownstream of the washing device, a stirred vessel for increasing thecontact and residence time between nitrated products and washing medium;(e) arranged in the production line downstream of the washing unit andthe optional stirred vessel if present, a separation device forseparating off the nitrated products which have been freed of theimpurities from the washing medium.
 2. The production plant as claimedin claim 1, wherein the dispersing device is a jet mixer or acentrifugal pump.
 3. The production plant as claimed in claim 1, whereinthe dispersing device is installed upstream of the reactor, with thedispersing device going over into the tube reactor or being integratedinto the tube reactor as a constituent of the tube reactor.
 4. Theproduction plant as claimed in claim 1, wherein the tube reactor isequipped with mixing elements for introduction of additional mixingenergy
 5. The production plant as claimed in claim 1, wherein the mixingelements are configured as plates, as impingement plates, as deflectionplates, as orifice plates, as static mixers or as flow dividers.
 6. Theproduction plant as claimed in claim 1, wherein the mixing elements areconfigured such to provide a mixing energy of from 10 to 1,000joule/liter.
 7. The production plant as claimed in claim 1, wherein themixing elements are configured such to provide a pressure drop permixing element in the range from 0.1 bar to 3.0 bar.